Nightmare Fuel is what it is, stuff that'll keep you awake at night or give you nightmares. High Octane stuff are things that while REALLY scare the living hell out of you and you'll pretty much be unable to sleep because of it. For better explainations, check out these articles:
http://tvtropes.org/pmwiki/pmwiki.php/Main/NightmareFuelhttp://tvtropes.org/pmwiki/pmwiki.php/Main/HighOctaneNightmareFuelSo, let's get straight to it. What do you consider N.F. and H.O.N.F. to you? As in, what stuff scares the living crap out of you and what stuff really scares you. Please refer to any media, from movies to comics.
For Nightmare Fuel, the Child Play series and The Drifting Classroom provide excellent scares and chills for me. Chucky and well... tons of the students in T.D.C. can really terrify me a lot.
However, my High Octane Nightmare Fuel is Uzumaki and that certain short story from Gyo... If you have read any of that stuff... you know why that really scares me. Want an example? Feel free to click the link below!
http://www.onemanga.com/Uzumaki/3/26/So, what's your nightmare fuel?